1. Field of the Invention
The present invention relates to an oscillator used for a television tuner etc. for receiving a wide frequency band signal.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing the basic configuration of this type of oscillator as a related art.
In FIG. 1, OSC represents an oscillation circuit, RSN a resonance circuit, MIX a mixing circuit, and AMP.sub.DC a DC amplifier.
The oscillation circuit OSC is integrated and is comprised of npn type transistors Q.sub.1 and Q.sub.2 comprising a differential type Colpitz oscillation circuit, npn type transistors Q.sub.3 and Q.sub.4 comprising a differential output stage, a biasing constant voltage source V.sub.1 of the oscillation npn type transistors Q.sub.1 and Q.sub.2, resistance elements R.sub.1 to R.sub.4, and constant current sources I.sub.1 to I.sub.3.
The base of the oscillation transistor Q.sub.1 is connected to an input/output terminal T.sub.1, is connected through the resistance element R.sub.1 to a line of a constant voltage source V.sub.1, and is connected through the resistance element R.sub.3 to the base of the transistor Q.sub.3. The emitter of the oscillation transistor Q.sub.1 is connected to an input/output terminal T.sub.2 and is connected to the constant current source I.sub.I, which constant current source I.sub.1 is grounded. The collector of the oscillation transistor Q.sub.1 is connected to a line of a power source voltage V.sub.cc.
The base of the oscillation transistor Q.sub.2 is connected to an input/output terminal T.sub.4, is connected through the resistance element R.sub.2 to the line of the constant voltage source V.sub.1, and is connected through the resistance element R.sub.4 to the base of the transistor Q.sub.4. The emitter of the oscillation transistor Q.sub.2 is connected to an input/output terminal T.sub.3 and is connected to the constant current source I.sub.3, which constant current source I.sub.2 is grounded. The collector of the oscillation transistor Q.sub.2 is connected to the line of the power source voltage V.sub.cc.
The transistors Q.sub.3 and Q.sub.4 are connected at their emitters and a node of the two is connected to the constant current source I.sub.3. The constant current source is grounded. The collectors of the transistors Q.sub.3 and Q.sub.4 are connected to the input of the mixing circuit MIX.
Further, the output terminal of the mixing circuit MIX is connected through the load resistance elements R.sub.5 and R.sub.6 to the line of the power source voltage V.sub.cc and is connected to the input of the DC amplifier AMP.sub.DC.
The resonance circuit RSN is comprised of a series circuit of a variable capacitive diode VC.sub.1 and coil L.sub.1 to which are connected in parallel the capacitors C.sub.1 and C.sub.2.
The node between one end of the coil L.sub.1 of the resonance circuit RSN and the capacitors C.sub.1 and C.sub.2 is grounded through the resistance element R.sub.7 of the resistance value of 30 k.OMEGA. and is connected to the input/output terminal T.sub.1 (base of oscillation transistor Q.sub.1) through the DC cutting capacitor C.sub.3.
The node between the cathode of the variable capacity diode VC.sub.1 and the capacitors C.sub.1 and C.sub.2 is connected to the output of the DC amplifier AMP.sub.DC through a drive resistance element R.sub.8 of a resistance value of 30 k.OMEGA. and is connected to an input/output terminal T.sub.4 (base of oscillation transistor Q.sub.2) through a DC cutting capacitor C.sub.3.
Between the node of the capacitor C.sub.3 and the input/output terminal T.sub.1 and the input/output terminal T.sub.2 is connected a positive feedback capacitor C.sub.5 and between the node of the capacitor C.sub.4 and the input/output terminal T.sub.4 is connected a positive feedback capacitor C.sub.6.
Further, between the node of the capacitor C.sub.5 and the input/output terminal T.sub.2 and the node of the capacitor C.sub.6 and the input/output terminal T.sub.3 (between the emitter of the oscillation transistor Q.sub.1 and the emitter of the oscillation transistor Q.sub.2) is connected a coupling capacitor C.sub.7.
Note that the capacities of the externally provided capacitors C.sub.1 to C.sub.7 are set for example as follows: 1 pF for the capacitor C.sub.1, 13 pF for the capacitor C.sub.2, 56 pF for the capacitors C.sub.3 and C.sub.4, 2 pF for the capacitors C.sub.5 and C.sub.6 and 3 pF for the capacitor C.sub.7.
In such a configuration, the oscillation circuit OSC receives positive feedback from the capacitors C.sub.5 and C.sub.6 connected between the bases and emitters of the oscillation transistors Q.sub.1 and Q.sub.2 through the input/output terminals T.sub.1, T.sub.2, T.sub.3, and T.sub.4, oscillates at the resonance frequency of the resonance circuit RSN connected to the bases of the oscillation transistors Q.sub.1 and Q.sub.2, and outputs the local oscillation frequency signal S.sub.L of a predetermined frequency through the transistors Q.sub.3 and Q.sub.4 to the mixing circuit MIX.
Note that the oscillation transistors Q.sub.1 and Q.sub.2 comprising the differential Colpitz oscillation circuit are connected at their bases through the resonance circuit RSN, so perform oscillation operations with opposite phases.
Accordingly, local oscillation frequency signals S.sub.L with opposite phases are output from the collectors of the transistors Q.sub.3 and Q.sub.4.
In the mixing circuit MIX, the FM modulated video signal of the selected channel and the local oscillation frequency signal S.sub.L are mixed, and the signal of the frequency of the difference is taken out and is output to the DC amplifier AMP.sub.DC. Further, the resonance circuit RSNa is driven through the coil L.sub.4 by the output of the DC amplifier AMP.sub.DC.
That is, after the phase detection by the mixing circuit MIX, the control voltage of the demodulation frequency, which is based on the drive resistance element R.sub.8 and the output of the DC amplifier AMP.sub.DC, is supplied to the cathode of the variable capacity diode VC.sub.1 of the resonance circuit RSN.
However, the above-mentioned circuit is used as an oscillator for an FM demodulator for satellite broadcasting (BS) and the frequency is 400 MHz to 500 MHz.
However, in the above-mentioned circuit, it suffers from the disadvantage that it is not possible to obtain a satisfactory picture since noise of a low frequency band would occur, which could be readily discerned even by the human eye, in the television picture, due to the so-called shot noise, flicker noise, or burst noise arising from the construction of the junction portion of the oscillation transistors Q.sub.1 and Q.sub.2 itself and the lattice faults, hot noise due to the resistance elements R.sub.1 and R.sub.2, etc.
Further, the control voltage of the demodulation frequency based on the output of the drive resistance element R.sub.8 and the DC amplifier AMP.sub.DC is supplied to the cathode of the variable capacity diode VC.sub.1 of the resonance circuit RSN, but the resistance value of the drive resistance element R.sub.8 is, as explained above, 30 k.OMEGA., the time constant with the capacitors is large, the drive impedance of the variable capacity diode VC.sub.1 is high, the frequency characteristics will not improved, and therefore this becomes a cause behind low frequency noise.
Since the operational frequency is a high of about 400 MHz to 500 MHz or so, the oscillation carrier easily flows into the power source and ground and therefore it suffers from the disadvantage that there is a liability of occurrence of a pseudo lock and a beat by the high frequency component.